Dry silver photographic materials have been known in the art since the late 1960's. Generally the negative acting light-sensitive layer is a coating comprising a binder, an image silver source, usually an organic silver salt such as silver behenate (often referred to as silver soap), a photocatalyst such as mercury halide, and a reducing agent for silver ion.
It is believed that when the photosensitive layer is exposed to light a latent image is formed in the silver halide (which may be produced by the reaction of the silver source material with a halogen compound or it may be introduced as preformed silver halide). If the photosensitive material is visible light sensitive, the halide is preferably bromide ion.
Thereafter the silver source material can be reduced by heating in the presence of the reducing agent, this reduction being catalyzed image-wise by the silver formed on the light exposed silver halide. By a suitable choice of temperature, the reduction of the silver compound can be catalyzed in the light exposed areas to give a visible darkening while any slight reduction which occurs in the non-light exposed areas is insufficient to give a marked visible change. Other components in the layer are solvents which enable the mixture to be coated upon a substrate, dyes, development accelerators which improve the definition of the photographic image, antihalation agents, and surfactants.
Light-stable sheet materials for recording light images have been described. U.S. Pat. No. 3,802,888 relates to an light-stable sheet material which requires preheating at temperatures in the range of 210.degree.-285.degree. F. (99.degree.-141.degree. C.) to render it light-sensitive. U.S. Pat. No. 3,764,329 discloses an initially substantially light-insensitive sheet material containing an organic haloamide compound as a halogen source, the material being rendered light-sensitive by heating at temperatures in the range of 90.degree.-143.degree. C. These two patents refer to photothermographic systems that are essentially light insensitive until heat activated, and are then light exposed and again heated to produce an image. Essentially, these patents relate to "add-on" photothermographic systems, which are useful, for example, when new graphic intelligence is to be added to an existing microfiche card or transparency.